In the related art, a semiconductor element such as a semiconductor laser diode or a photodiode, and a semiconductor element housing package for housing the semiconductor element, which are used in fields of optical communication and high-speed signal process, are known. A semiconductor device is configured by housing the semiconductor element inside such a package and electrically connecting the semiconductor element to the outside of the package for assembling. Moreover, a semiconductor device in which lead pins are provided along one side of a lower surface of the package housing the semiconductor element is suggested (refer to Japanese Unexamined Utility Model Publication JP-U 3-88347 (1991)).
When using a small mounting substrate in which a width of one side of an external mounting substrate does not exceed a width of one side of the semiconductor device, it is not possible to provide the lead pins on two or more sides of the lower surface of the semiconductor device. For example, if the lead pins are provided on all of four sides of the lower surface of the semiconductor device, a pitch of a signal line provided on the mounting substrate becomes a narrow pitch and a routing pattern of the signal line on the mounting substrate is complicated. Furthermore, the lead pins are more likely to be electrically connected to an unintended signal line and manufacturing yield may be degraded.
However, the semiconductor element housing package has a structure in which, if the lead pins are simply provided along one side of the lower surface, heat is transmitted from the semiconductor element to the lead pins and thermal expansion is caused in the lead pins or in a bonding material that bonds the lead pins and an external substrate in a state where the lead pins are connected to the external substrate so that the package is likely to be inclined.
An object of the invention is to provide a semiconductor element housing package capable of reducing inclination of a package even if heat is transmitted from a semiconductor element to the lead pins, a semiconductor device, and a mounting structure.